The meaning of bravery is an important theme of the novel. At the beginning, Daniel often seems to be more reckless than brave. He is brave when he does the right thing out of goodness but less brave when he does it because Rosh orders him or out of hatred and revenge. For example, he is brave when he disobeys Rosh and helps the man he has just beaten and taken money from, and he is even brave when he risks the ridicule of the village by getting water and food (women's work) to care for his sister. When he spits at a Roman soldier or refuses to pick up a bundle when required by Roman law, he is more reckless than brave, especially because he endangers Thacia. Joel does not act out of selfish reasons, however. His bravery arises from love of country and desire to help others rather than a personal hatred for personal injuries. Often Joel shows himself ready to sacrifice personal interest, not out of bravado (which often motivates Daniel), but because he thinks it is the best thing to do.
I would say both are equally brave. For example, Daniel will openly defy Roman soliders, throwing water in one's face and taunting others even though he certainly knows there will be physical retribution. Joel is brave in that he chooses to disobey his father and help Daniel on a quest he has embraced as his own: ousting the Roman rule. It is brave because Joel and his family have a comfortable live and he is putting that ease at risk.
Yes, both boys are brave (and Thacia too) but they must learn to channel their passions productively. Eventually Joel and Daniel learn that Jesus' path of love and non-violent resistance is the only true bravery that will be successful in the face of hatred and oppression.